package com.samxcode.leetcode;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d =
 * target? Find all unique quadruplets in the array which gives the sum of target.
 * 
 * Note: Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
 * The solution set must not contain duplicate quadruplets.
 * 
 * Solution:
 * Based on the solution of 3Sum
 * 
 * @author Sam
 *
 */
public class FourSum {
    
    public static List<List<Integer>> fourSum(int[] num, int target) {
        List<List<Integer>> res = new ArrayList<>();
        if (num.length < 4) {
            return res;
        }
        Arrays.sort(num);
        for (int i = 0; i < num.length - 3; i++) {
            if (i > 0 && num[i] == num[i - 1]) {
                continue;
            }
            for (int j = i + 1; j < num.length - 2; j++) {
                if (j > i + 1 && num[j] == num[j - 1]) {
                    continue;
                }
                int left = j + 1, right = num.length - 1;
                while (left < right) {
                    int sum = num[i] + num[j] + num[left] + num[right];
                    if (sum > target) {
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        res.add(Arrays.asList(num[i], num[j], num[left], num[right]));
                        while (left < right && num[right] == num[--right]);
                        while (left < right && num[left] == num[++left]);
                    }
                }
            }
        }
        return res;
    }
}
